The Demise of the Hemlocks
The Eastern Hemlock plays an important role in the ecology and hydrology of mountain ecosystems. Hemlocks provide critical habitat for birds and other animals; their shade helps maintain the cool water temperatures required by trout and other aquatic organisms in mountain streams.
The tall, long-lived hemlock, which shelters river and streamside ecosystems throughout the eastern United States and Canada, is rapidly declining in number due to a Japanese aphid-like insect known as Adelges tsugae, or hemlock woolly adelgid (HWA). These insects kill the hemlocks by sucking them dry of sap while concealing themselves in tiny cotton masses.
A white, woolly material near the base of hemlock needles may not seem like a dangerous plant feeder, but this sure sign of HWA infestation and brings death to eastern hemlocks and drastic change to Virginia's ecosystem.
American scientists first noticed the tiny insect in the Western Asia as early as the 1920s, but it wasn't seen on the East Coast until the 1950s when the cotton egg masses were found in ornamental plantings near Richmond, Virginia. According to the state Division of Forestry, the adelgid, a native of Japan, is spreading along the eastern United States, carried by wind or on birds and animals.
How it found it's way to the United States is a mystery, it could have can in through lumber or furniture undetected. In 2006, scientists performed a series of DNA comparisons of hemlock woolly adelgid throughout its current geographic range, including China, Japan, western North America, and eastern North America. They found an exact match between the HWA in eastern North America with the Osaka region of Japan. The hemlocks in Western Asia co-evolved with the adelgid species and are resistant to them. Our hemlocks in the East never developed a natural immunity to the insect. The early infestation in rural Virginia's communities was dealt with using insecticides and not considered to be a significant threat. Once the insects spread to Virginia vast Shenandoah Valley forest the true threat was realized.
The hemlock woolly adelgid (HWA) has killed more than 90% of the hemlocks in the Shenandoah Valley and spread at an average rate of 15 miles per year - or faster in warm, wet climates along the coast or other bodies of water. They are consuming hemlocks from Georgia to Maine. Without any natural controls, such as predators or disease, HWA populations will continue to prey on American forests until few, if any, hemlocks remain. The HWA now cover more than 50 percent of the geographic range of eastern hemlocks. Cold, winter temperatures inhibit the insect's ability to travel, slowing its expansion rate.
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